It has happened in multiple tanks, with java fern sourced from different suppliers. It's always the java fern that starts it. If I have anubias in the tank by itself, it is fine. And it can happens weeks or months after I've had the java fern in my tank. I had this java fern as healthy looking as the one in your photo, and then in a couple of days, the whole plant had melted. Also the rhizome is always healthy. It's just the leaves that get infected and then die back.
Reading online it seems some people think there might be some sort of disease, or it might have something to do with CO2 levels. However, the way it spreads to other rhizome plants in the tank makes me think it might be disease related.
It's a shame as I love to use java fern in my wild betta tanks.